Bile acids are the terminal metabolites of hepatic cholesterol and assist in the solubilization of intestinal lipids and fat-soluble vitamins. In addition, through their binding to the nuclear receptor FXR, certain bile acids can regulate the expression of genes responsible for their biosynthesis and transport. FXR acts as the liver's sensor of bile acid pools, binding preferentially to chenodeoxycholic acid (CDCA), and transcriptionally regulating genes responsible for cholesterol and bile acid homeostasis. This proposal relies on structural, biochemical and cell-based transcriptional reporter assays to decipher the interactions of FXR with its ligands, and to characterize the receptor conformations responsible for both coactivator and corepressor binding. In the first four aims, we rely on the isolated ligand binding domain (LBD) of FXR and propose a set of experiments to provide stereochemical information about how FXR recognizes both steroidal and nonsteroidal ligands, and how ligand binding alters the receptor's affinities for coactivator derived LXXLL containing motifs and corepressor-derived (I/L)XXII motifs. We also propose to use mutations of FXR at critical functional sites, as well as novel chemical entities related to bile acids, to further expand and test our notions of how FXR forms its molecular interactions. In the fifth aim, we propose to determine the crystal structure of the FXR-RXR heterodimer on its cognate DNA response element, in order to understand how RXR and DNA binding further impact FXR's functional surfaces. [unreadable] [unreadable]